1. Field of the Invention
The present invention relates to a bandpass filter using a surface acoustic wave resonator, and more particularly, to a surface acoustic wave resonator filter including a plurality of longitudinally coupled surface acoustic wave resonator filters which are connected in parallel.
2. Description of the Related Art
Surface acoustic wave resonator filters are now widely used as bandpass filters in mobile communication devices. In recent years, digital signal transmission techniques have become popular in mobile communication systems. However, unlike analog mobile communication systems, it is strongly required that the filters used in the digital mobile communication systems have not only high temperature stability and a sharp cutoff characteristic at band edges but also a wide bandwidth and a flat group delay time characteristic over the entire passband. In particular, these requirements are critical for an intermediate frequency (IF) filter.
Unlike the analog mobile communication systems in which surface acoustic wave filters consisting of transversely coupled surface acoustic wave resonator filters are usually used, transversal surface acoustic wave filters or longitudinally coupled surface acoustic wave resonator filters are used in the digital mobile communication systems in order to meet the above requirements.
Although transversal surface acoustic wave filters have the advantage that a wide bandwidth and a flat group delay time characteristic can be easily achieved, they have the disadvantage that they have a large insertion loss and it is difficult to produce such filters to have a relatively small size.
On the other hand, the longitudinally coupled surface acoustic wave resonator filter has a lower insertion loss than the transversal surface acoustic wave filter and can have a wider bandwidth than the transversely coupled surface acoustic wave resonator filter. However, the disadvantage of the longitudinally coupled surface acoustic wave resonator filter is that the attenuation at the higher edge of the passband is not sharp, as shown in FIG. 5. In FIG. 5, the solid line A represents the same characteristic as the solid line B but the attenuation is expressed in an expanded scale denoted on the right of the vertical axis.
To avoid the above problem, it has been proposed to connect in parallel a plurality of longitudinally coupled resonator filters having different passbands thereby realizing a surface acoustic wave resonator filter having a wide bandwidth and having an improved attenuation characteristic at the higher edge of the passband as shown in FIG. 6 (refer to, for example, Japanese Patent Laid-Open No. 6-334476). In this specific example, a lower-frequency longitudinally coupled surface acoustic wave resonator filter having a relatively wide bandwidth is connected in parallel to a higher-frequency longitudinally coupled surface acoustic wave resonator filter having a relatively narrow bandwidth.
However, as described in Japanese Patent Laid-Open No. 6-334476, the attenuation characteristic at the higher band edge of this surface acoustic wave resonator filter is realized merely by the attenuation characteristic of the higher-frequency, narrow-bandwidth longitudinally coupled surface acoustic wave resonator filter, and thus the attenuation characteristic at the higher edge of the passband is not sufficient.
Furthermore, in the technique disclosed in Japanese Patent Laid-Open No. 334476, the resonance mode of each longitudinally coupled surface acoustic wave resonator filter is set in such a manner that the resonance modes of higher-frequency and lower-frequency longitudinally coupled surface acoustic wave resonator filters overlap with each other so that the output signals of these two longitudinally coupled surface acoustic wave resonator filters are always equal in phase thereby defining the overall filter characteristic.
However, in such a surface acoustic wave resonator filter, an inversion in phase occurs at the resonance frequency of each resonance mode. Therefore, if there is a deviation in frequency between the lower-frequency surface acoustic wave resonator filter and the higher-frequency surface acoustic wave resonator filter, a frequency range appears where the output signals of the two surface acoustic wave resonator filters become nearly equal in level but opposite in phase. This means that a slight error in production processing such as a variation in the thickness or a width of an electrode film can cause such a deviation in frequency which produces a range where the output signals become equal in level but opposite in phase. The emergence of such a range can cause a group delay ripple C near the center of the passband, as shown in FIG. 7. Such a group delay ripple C causes great degradation in the characteristics of the IF filter used in the digital communication device.